Electrospinning Process

ABSTRACT

A method for electrospinning polymer fibers and the resultant electrospun fibers are disclosed. In the electrospinning method, the polymer undergoes a crosslinking reaction prior to and during the electrospinning process.

FIELD OF THE INVENTION

The present invention relates to an electrospinning process, theresulting electrospun fiber and polymers used in the electrospinningprocess.

BACKGROUND OF THE INVENTION

The process of electrospinning uses an electrical charge to form finefibers. The process consists of a spinneret with a dispensing needle, asyringe pump, a power supply and a grounded collection device. Polymersin solution or as melts are located in the syringe and driven to theneedle tip by the syringe pump where they form a droplet. When voltageis applied to the needle, a droplet is stretched to an electrifiedliquid jet. The jet is elongated continuously until it is deposited onthe collector as a mat of fine fibers usually of nanometer-sizeddimensions. The resultant fibers are useful in a wide variety ofapplications such as protective clothing, wound dressing and as supportsor carriers for catalyst. To form a fiber, the polymeric melt orsolution must have a sufficient viscosity otherwise a drop rather than aliquid jet will form. Typically, thickeners are included in the polymersolution or melt to provide the necessary viscosity. However, thickenerscan adversely affect the properties of the resultant fibers and for thisreason, their use should be minimized.

SUMMARY OF THE INVENTION

The present invention provides for a process of electrospinning a fiberfrom an electrically conductive solution of a polymer in the presence ofan electric field between a spinneret and a ground source. The polymerundergoes a crosslinking reaction prior to and during theelectrospinning process resulting in a viscosity buildup of the polymersolution enabling fiber formation and minimizing the use of thickeners.

The invention also provides for the resultant electrospun fiber thatcontains silane, preferably carboxyl and hydroxyl groups and optionallya nitrogen-containing group such as amine or amide groups. The silanegroups provide for crosslinking and viscosity build-up. The carboxyl,hydroxyl, amine and amide groups provide for a hydrogen bonding andviscosity build-up. The carboxyl group, in the form of carboxylic acid,and the nitrogen-containing groups are good electrical charge carryinggroups.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a basic electrospinning system.

FIG. 2 simulates a scanning electron microscopic (SCM) image of anon-woven mat.

DETAILED DESCRIPTION OF THE INVENTION

For purposes of the following detailed description, it is to beunderstood that the invention may assume various alternative variationsand step sequences, except where expressly specified to the contrary.Moreover, other than in any operating examples, or where otherwiseindicated, all numbers expressing, for example, quantities ofingredients used in the specification and claims are to be understood asbeing modified in all instances by the term “about”. Accordingly, unlessindicated to the contrary, the numerical parameters set forth in thefollowing specification and attached claims are approximations that mayvary depending upon the desired properties to be obtained by the presentinvention. At the very least, and not as an attempt to limit theapplication of the doctrine of equivalents to the scope of the claims,each numerical parameter should at least be construed in light of thenumber of reported significant digits and by applying ordinary roundingtechniques. Notwithstanding that the numerical ranges and parameterssetting forth the broad scope of the invention are approximations, thenumerical values set forth in the specific examples are reported asprecisely as possible. Any numerical value, however, inherently containscertain errors necessarily resulting from the standard variation foundin their respective testing measurements.

Also, it should be understood that any numerical range recited herein isintended to include all sub-ranges subsumed therein. For example, arange of “1 to 10” is intended to include all sub-ranges between (andincluding) the recited minimum value of 1 and the recited maximum valueof 10, that is, having a minimum value equal to or greater than 1 and amaximum value of equal to or less than 10.

In this application, the use of the singular includes the plural andplural encompasses singular, unless specifically stated otherwise. Inaddition, in this application, the use of “or” means “and/or” unlessspecifically stated otherwise, even though “and/or” may be explicitlyused in certain instances.

The term “polymer” is also meant to include copolymer and oligiomer. Theterm “acrylic” is meant to include methacrylic and is depicted by(meth)acrylic.

With reference to FIG. 1, the electrospinning system consists of threemajor components, a power supply 1, a spinneret 3 and an electricallygrounded collector 4. Direct current or alternating current may be usedin the electrospinning process. The polymer solution 5 is contained in asyringe 7. A syringe pump 9 forces the solution through the spinneret 3at a controlled rate. A drop of the solution forms at the tip of theneedle 11. Upon application of a voltage, typically from 5 to 30kilovolts (kV), the drop becomes electrically charged. Consequently, thedrop experiences electrostatic repulsion between the surface charges andthe forces exerted by the external electric field. These electricalforces will distort the drop and will eventually overcome the surfacetension of the polymer solution resulting in the ejection of a liquidjet 13 from the tip of the needle 11. Because of its charge, the jet isdrawn downward to the grounded collector 4. During its travel towardsthe collector 4, the jet 13 undergoes a stretching action leading to theformation of a thin fiber. The charged fiber is deposited on thecollector 4 as a random oriented non-woven mat as generally shown inFIG. 2.

The polymers of the present invention can be acrylic polymers. As usedherein, the term “acrylic” polymer refers to those polymers that arewell known to those skilled in the art which results in thepolymerization of one or more ethylenically unsaturated polymerizablematerials. (Meth)acrylic polymers suitable for use in the presentinvention can be made by any of a wide variety of methods as will beunderstood by those skilled in the art. The (meth)acrylic polymers canbe made by addition polymerization of unsaturated polymerizablematerials that contain silane groups, carboxyl groups, hydroxyl groupsand optionally a nitrogen-containing group. Examples of silane groupsinclude, without limitation, groups that have the structure Si—X_(n)(wherein n is an integer having a value ranging from 1 to 3 and X isselected from chlorine, alkoxy esters, and/or acyloxy esters). Suchgroups hydrolyze in the presence of water including moisture in the airto form silanol groups that condense to form —Si—O—Si— groups.

Examples of silane-containing ethylenically unsaturated polymerizablematerials, suitable for use in preparing such (meth)acrylic polymersinclude, without limitation, ethylenically unsaturated alkoxy silanesand ethylenically unsaturated acyloxy silanes, more specific examples ofwhich include vinyl silanes such as vinyl trimethoxysilane,acrylatoalkoxysilanes, such as gamma-acryloxypropyl trimethoxysilane andgamma-acryloxypropyl triethoxysilane, and methacrylatoalkoxysilanes,such as gamma-methacryloxypropyl trimethoxysilane,gamma-methacryloxypropyl triethoxysilane and gamma-methacryloxypropyltris-(2-methoxyethoxy) silane; acyloxysilanes, including, for example,acrylato acetoxysilanes, methacrylato acetoxysilanes and ethylenicallyunsaturated acetoxysilanes, such as acrylatopropyl triacetoxysilane andmethacrylatopropyl triacetoxysilane. In certain embodiments, it may bedesirable to utilize monomers that, upon addition polymerization, willresult in a (meth)acrylic polymer in which the Si atoms of the resultinghydrolyzable silyl groups are separated by at least two atoms from thebackbone of the polymer. Preferred monomers are(meth)acryloxyalkylpolyalkoxy silane, particularly(meth)acryloxyalkyltrialkoxy silane in which the alkyl group containsfrom 2 to 3 carbon atoms and the alkoxy groups contain from 1 to 2carbon atoms.

In certain embodiments, the amount of the silane-containingethylenically unsaturated polymerizable material used in the totalmonomer mixture is chosen so as to result in the production of a(meth)acrylic polymer comprising silane groups that contain from 0.2 to20, preferably 5 to 10 percent by weight, silicon, based on the weightof the total monomer combination used in preparing the (meth)acrylicpolymer.

The (meth)acrylic polymer suitable for use in the present invention canbe the reaction product of one or more of the aforementionedsilane-containing ethylenically unsaturated polymerizable materials andpreferably an ethylenically unsaturated polymerizable material thatcomprises carboxyl such as carboxylic acid groups or an anhydridethereof. Examples of suitable ethylenically unsaturated acids and/oranhydrides thereof include, without limitation, acrylic acid,methacrylic acid, itaconic acid, crotonic acid, maleic acid, maleicanhydride, citraconic anhydride, itaconic anhydride, ethylenicallyunsaturated sulfonic acids and/or anhydrides such as sulfoethylmethacrylate, and half esters of maleic and fumaric acids, such as butylhydrogen maleate and ethyl hydrogen fumarate in which one carboxyl groupis esterified with an alcohol.

Examples of other polymerizable ethylenically unsaturated monomers tointroduce carboxyl functionality are alkyl including cycloalkyl andaryl(meth)acrylates containing from 1 to 12 carbon atoms in the alkylgroup and from 6 to 12 carbon atoms in the aryl group. Specific examplesof such monomers include methyl methacrylate, n-butyl methacrylate,n-butyl acrylate, 2-ethylhexyl methacrylate, cyclohexyl methacrylate andphenyl methacrylate.

The amount of the polymerizable carboxyl-containing ethylenicallyunsaturated monomers is preferably sufficient to provide a carboxylcontent of up to 55, preferably 15.0 to 45.0 percent by weight based onthe weight of the total monomer combination used to prepare the(meth)acrylic polymer. Preferably, at least a portion of the carboxylgroups are derived from a carboxylic acid such that the acid value ofthe polymer is within the range of 20 to 80, preferably 30 to 70, on a100% resin solids basis.

The (meth)acrylic polymer used in the invention also preferably containshydroxyl functionality typically achieved by using a hydroxyl functionalethylenically unsaturated polymerizable monomer. Examples of suchmaterials include hydroxyalkyl esters of (meth)acrylic acids having from2 to 4 carbon atoms in the hydroxyalkyl group. Specific examples includehydroxyethyl(meth)acrylate, hydroxypropyl(meth)acrylate and4-hydroxybutyl (meth)acrylate. The amount of the hydroxy functionalethylenically unsaturated monomer is sufficient to provide a hydroxylcontent of up to 6.5 such as 0.5 to 6.5, preferably 1 to 4 percent byweight based on the weight of the total monomer combination used toprepare the (meth)acrylic polymer.

The (meth)acrylic polymer optionally contains nitrogen functionalityintroduced from a nitrogen-containing ethylenically unsaturated monomer.Examples of nitrogen functionality are amines, amides, ureas, imidazolesand pyrrolidones. Examples of suitable N-containing ethylenicallyunsaturated monomers are: amino-functional ethylenically unsaturatedpolymerizable materials that include, without limitation,p-dimethylamino ethyl styrene, t-butylaminoethyl(meth)acrylate,dimethylaminoethyl(meth)acrylate, diethylaminoethyl(meth)acrylate,dimethylaminopropyl(meth)acrylate anddimethylaminopropyl(meth)acrylamide; amido-functional ethylenicallyunsaturated materials that include acrylamide, methacrylamide, n-methylacrylamide and n-ethyl(meth)acrylamide; urea functional ethylenicallyunsaturated monomers that include methacrylamidoethylethylene urea.

If used, the amount of the nitrogen-containing ethylenically unsaturatedmonomer is sufficient to provide nitrogen content of up to 5 such asfrom 0.2 to 5.0, preferably from 0.4 to 2.5 percent by weight based onweight of a total monomer combination used in preparing the(meth)acrylic polymer.

Besides the polymerizable monomers mentioned above, other polymerizableethylenically unsaturated monomers that may be used to prepare the(meth)acrylic polymer. Examples of such monomers includepoly(meth)acrylates such as ethylene glycol di(meth)acrylate,trimethylolpropane tri(meth)acrylate, ditrimethylolpropanetetraacrylate; aromatic vinyl monomers such as styrene, vinyl tolueneand alpha-methylstyrene; monoolefinic and diolefinic hydrocarbons,unsaturated esters of organic and inorganic acids and esters ofunsaturated acids and nitrites. Examples of such monomers include1,3-butadiene, acrylonitrile, vinyl butyrate, vinyl acetate, allylchloride, divinyl benzene, diallyl itaconate, triallyl cyanurate as wellas mixtures thereof. The polyfunctional monomers, such as thepolyacrylates, if present, are typically used in amounts up to 20percent by weight. The monfunctional monomers, if present, are used inamount up to 70 percent by weight; the percentage being based on weightof the total monomer combination used to prepare the (meth)acrylicpolymer.

The (meth)acrylic polymer is typically formed by solution polymerizationof the ethylenically unsaturated polymerizable monomers in the presenceof a polymerization initiator such as azo compounds, such as alpha,alpha′-azobis(isobutyronitrile), 2,2′-azobis(methylbutyronitrile) and2,2′-azobis(2,4-dimethylvaleronitrile); peroxides, such as benzoylperoxide, cumene hydroperoxide and t-amylperoxy-2-ethylhexanoate;tertiary butyl peracetate; tertiary butyl perbenzoate; isopropylpercarbonate; butyl isopropyl peroxy carbonate; and similar compounds.The quantity of initiator employed can be varied considerably; however,in most instances, it is desirable to utilize from 0.1 to 10 percent byweight of initiator based on the total weight of copolymerizablemonomers employed. A chain modifying agent or chain transfer agent maybe added to the polymerization mixture. The mercaptans, such as dodecylmercaptan, tertiary dodecyl mercaptan, octyl mercaptan, hexyl mercaptanand the mercaptoalkyl trialkoxysilanes such as 3-mercaptopropyltrimethoxysilane may be used for this purpose as well as other chaintransfer agents such as cyclopentadiene, allyl acetate, allyl carbamate,and mercaptoethanol.

The polymerization reaction for the mixture of monomers to prepare theacrylic polymer can be carried out in an organic solvent mediumutilizing conventional solution polymerization procedures which are wellknown in the addition polymer art as illustrated with particularity in,for example, U.S. Pat. Nos. 2,978,437; 3,079,434 and 3,307,963. Organicsolvents that may be utilized in the polymerization of the monomersinclude virtually any of the organic solvents often employed inpreparing acrylic or vinyl polymers such as, for example, alcohols,ketones, aromatic hydrocarbons or mixtures thereof. Illustrative oforganic solvents of the above type which may be employed are alcoholssuch as lower alkanols containing 2 to 4 carbon atoms including ethanol,propanol, isopropanol, and butanol; ether alcohols such as ethyleneglycol monoethyl ether, ethylene glycol monobutyl ether, propyleneglycol monomethyl ether, and dipropylene glycol monoethyl ether; ketonessuch as methyl ethyl ketone, methyl N-butyl ketone, and methyl isobutylketone; esters such as butyl acetate; and aromatic hydrocarbons such asxylene, toluene, and naphtha.

In certain embodiments, the polymerization of the ethylenicallyunsaturated components is conducted at from 0° C. to 150° C., such asfrom 50° C. to 150° C., or, in some cases, from 80° C. to 120° C.

The polymer prepared as described above is usually dissolved in solventand typically has a resin solids content of about 15 to 80, preferably20 to 60 percent by weight based on total solution weight. The molecularweight of the polymer typically ranges between 3,000 to 1,000,000,preferably 5,000 to 100,000 as determined by gel permeationchromatography using a polystyrene standard.

For the electrospinning application, the polymer solution such asdescribed above can be mixed with water to initiate the crosslinkingreaction and to build viscosity necessary for fiber formation. Typicallyabout 5 to 20, preferably 10 to 15 percent by weight water is added tothe polymer solution with the percentage by weight being based on totalweight of the polymer solution and the water. Preferably a base such asa water-soluble organic amine is added to the water-polymer solution tocatalyze the crosslinking reaction. Optionally a thickener such aspolyvinyl pyrrolidone, polyvinyl alcohol, polyvinyl acetate, polyamidesand/or a cellulosic thickener can be added to the electrospinningformulation to better control its viscoelastic behavior. If used, thethickener is present in amounts no greater than 20 percent by weight,typically from 1 to 6 percent by weight based on weight of the polymersolution.

The electrospinning formulation prepared as described above is thenstored to permit the viscosity to build to the crosslinking reaction.When the viscosity is sufficiently high but short of gelation, theformulation is subjected to the electrospinning process as describedabove.

Typically, the viscosity is at least 5 and less than 2,000, usually lessthan 1,000, such as preferably within the range of 50 to 250 centistokesfor the electrospinning process. A Bubble Viscometer according to ASTMD-1544 determines the viscosity. The time for storing theelectrospinning formulation will depend on a number of factors such astemperature, crosslinking functionality and catalyst. Typically, theelectrospinning formulation will be stored for as low as one minute upto two hours.

When subject to the electrospinning process, the formulations describedabove typically produce fibers having a diameter of up to 5,000, such asfrom 5 to 5,000 nanometers, more typically within the range of 50 to1,200 nanometers, such as 50 to 700 nanometers. The fibers also can havea ribbon configuration and in this case diameter is intended to mean thelargest dimension of the fiber. Typically the width of the ribbon shapedfibers is up to 5000 such as 500 to 5000 nanometers and the thickness upto 200 such as 5 to 200 nanometers.

The following examples are presented to demonstrate the generalprinciples of the invention. However, the invention should not beconsidered as limited to the specific examples presented. All parts areby weight unless otherwise indicated.

EXAMPLES A, B and C Synthesis of Acrylic Silane Polymers

For each of Examples A to C in Table 1 below, a reaction flask wasequipped with a stirrer, thermocouple, nitrogen inlet and a condenser.Charge A was then added and stirred with heat to reflux temperature (75°C.-80° C.) under nitrogen atmosphere. To the refluxing ethanol, charge Band charge C were simultaneously added over three hours. The reactionmixture was held at reflux condition for two hours. Charge D was thenadded over a period of 30 minutes. The reaction mixture was held atreflux condition for two hours and subsequently cooled to 30° C.

TABLE 1 Example A Example B Example C Charge A (weight in grams) EthanolSDA 40B¹ 360.1  752.8 1440.2 Charge B (weight in grams) MethylMethacrylate 12.8  41.8 137.9 Acrylic acid 8.7  18.1 34.6 SilquestA-174² 101.4  211.9 405.4 2-hydroxylethylmethacrylate 14.5   0.3 0.64n-Butyl acrylate 0.2   0.3 0.64 Acrylamide 7.2 — — Sartomer SR 355³ — 30.3 — Ethanol SDA 40B 155.7  325.5 622.6 Charge C (weight in grams)Vazo 67⁴ 6.1  12.8 24.5 Ethanol SDA 40B 76.7  160.4 306.8 Charge D(weight in grams) Vazo 67 1.5   2.1 6.1 Ethanol SDA 40B 9.1  18.9 36.2 %Solids 17.9  19.5 19.1 Acid value 51.96  45.64 45.03 (100% resin solids)Mn — 3021⁵ 5810 ¹Denatured ethyl alcohol, 200 proof, available fromArcher Daniel Midland Co. ²gamma-methacryloxypropyltrimethoxysilane,available from GE silicones. ³Di-trimethylolpropane tetraacrylate,available from Sartomer Company Inc. ⁴2,2′-azo bis(2-methylbutyronitrile), available from E.I. duPont de Nemours & Co., Inc. ⁵Mn ofsoluble portion; the polymer is not completely soluble intetrahydrofuran.

EXAMPLES 1, 2 AND 3 Acrylic-Silane Nanofibers Example 1

The acrylic-silane resin solution from Example C (8.5 grams) was blendedwith polyvinylpyrrolidone (0.2 grams) and water (1.5 grams). Theformulation was stored at room temperature for 215 minutes. A portion ofthe resulting formulation was loaded into a 10 ml syringe and deliveredvia a syringe pump at a rate of 1.6 milliliters per hour to a spinneret(stainless steel tube 1/16-inch outer diameter and 0.010-inch internaldiameter). This tube was connected to a grounding aluminum collector viaa high voltage source to which about 21 kV potential was applied. Thedelivery tube and collector were encased in a box that allowed nitrogenpurging to maintain a relative humidity of less than 25%. Ribbon shapednanofibers having a thickness of about 100-200 nanometers and a width of500-700 nanometers were collected on the grounded aluminum panels andwere characterized by optical microscopy and scanning electronmicroscopy.

Example 2

The acrylic-silane resin solution from Example B (8.5 grams) was blendedwith polyvinylpyrrolidone (0.1 grams) and water (1.5 grams). Theformulation was stored at room temperature for 210 minutes. A portion ofthe resulting solution was loaded into a 10 ml syringe and delivered viaa syringe pump at a rate of 0.2 milliliters per hour to the spinneret ofExample 1. The conditions for electrospinning were as described inExample 1. Ribbon shaped nanofibers having a thickness of 100-200nanometers and a width of 900-1200 nanometers were collected on groundedaluminum foil and were characterized by optical microscopy and scanningelectron microscopy.

Example 3

The acrylic-silane resin from Example A (8.5 grams) was blended withpolyvinylpyrrolidone (0.1 grams) and water (1.5 grams). The formulationwas stored at room temperature for 225 minutes. A portion of theresulting solution was loaded into a 10 ml syringe and delivered via asyringe pump at a rate of 1.6 milliliters per hour to the spinneret asdescribed in Example 1. The conditions for electrospinning were asdescribed in Example 1. Ribbon shaped nanofibers having a thickness of100-200 nanometers and a width of 1200-5000 nanometers were collected ongrounded aluminum foil and were characterized by optical microscopy andscanning electron microscopy. A sample of the nanofibers was dried in anoven at 110° C. for two hours. No measurable weight loss was observed.This indicates the nanofibers were completely crosslinked.

Whereas particular embodiments of this invention have been describedabove for purposes of illustration, it will be evident to those skilledin the art that numerous variations of the details of the presentinvention may be made without departing from the invention as defined inthe appended claims.

1. A method for electrospinning a fiber from an electrically conductingsolution of polymer in the presence of an electric field between aspinneret and a ground source, the polymer undergoing a crosslinkingreaction prior to and during electrospinning.
 2. The method of claim 1in which the polymer contains crosslinkable groups along the polymerbackbone.
 3. The method of claim 2 in which the crosslinkable groups arereactive with moisture.
 4. The method of claim 3 in which thecrosslinkable groups are silane groups.
 5. The method of claim 2 inwhich the polymer is a (meth)acrylic polymer.
 6. The method of claim 2in which the polymer is a (meth)acrylic polymer containing silanegroups.
 7. The method of claim 2 in which the polymer, besidescontaining crosslinkable groups, also contains groups selected fromcarboxyl and hydroxyl.
 8. The method of claim 2 in which the polymercontains silane groups, carboxyl groups, hydroxyl groups andnitrogen-containing groups.
 9. The method of claim 2 in which the silanegroups are present in the polymer in amounts of 0.2 to 20 percent byweight silicon based on total polymer weight.
 10. The method of claim 8in which the polymer contains from: (a) 0.2 to 20 percent silane groupmeasured as silicon, (b) 1 to 45 percent carboxyl groups, (c) 0.5 to 6.5percent hydroxyl groups, and (d) 0.2 to 5.0 percent nitrogen groups; thepercentages by weight being based on total polymer weight.
 11. Themethod of claim 1 in which the solution contains a thickener.
 12. Themethod of claim 11 in which the thickener is polyvinyl pyrrolidone. 13.The method of claim 12 in which the polyvinyl pyrrolidone is present inamounts of no greater than 20 percent by weight based on total weight ofsolution.
 14. An electrospun fiber comprising a polymer that has beencrosslinked prior to and during the electrospinning process.
 15. Theelectrospun fiber of claim 14 having a diameter of from 5 to 5,000nanometers.
 16. The electrospun fiber of claim 14 having —Si—O—Si—crosslinks.
 17. The electrospun fiber of claim 14 being a crosslinked(meth)acrylic polymer.
 18. The electrospun fiber of claim 14 being a(meth)acrylic polymer with —Si—O—Si— crosslinks.